There are definitely some people who are taking the piss, bringing on suitcases so vast that there’s NO WAY they weigh less than 7kg. Some of them get stopped at the door of the plane and relieved of their luggage (which gets sent for check-in), so why they don’t just deal with it at the desk is a bit of a mystery.
This one varies by nationality. It still makes me laugh when I go to the Philippines and the Filipinos are all diving for the luggage lockers while the “please stay in your seats” announcement plays over the mayhem. They don’t seem to realise they can’t get off the plane until the plane stops and the door opens. I suspect they’re just suffering from Facebook withdrawal and want to log on ASAP.